Pigmented powder coating compositions that meet a customer's colour specifications require a labour-intensive multi-step approach during their preparation, for example, batch mixing, extrusion, further processing and coating of powder samples, measurements of colour, laboratory tests, adjustment of the raw materials and re-mixing and re-processing, re-mixing with extra pigment. In general, it is difficult to achieve the desired colour without correction steps because of batch to batch pigment variability and equipment performance variability.
One way to solve this problem is the provision of a limited number of stored, intermediate coating compositions which are combined, depending on the desired colour effect, and corresponding to the specific customer needs. Thus, there exists in the literature a number of examples describing dry-blends of powder coating compositions by mixing powder coating intermediate compositions having different colours, for example, WO 99/50360, EP-A 826 746, JP-A 11279464, JP-A 11286635.
WO 98/36030 describes a coloured powder coating composition consisting of two or more colour formulations which are dry-mixed together. The particles of one formulation are coloured and the particles of the other formulation have a different colour, or are optionally colourless. The composition forms a continuous coating, and the differences in colour cannot be differentiated by human eye.
WO 99/23068 discloses a process wherein different coloured fine powder compositions are compacted resulting in powders of uniform colours.
JP-A 52-47 031 describes the production of thermosetting powder paints, whereby the thermosetting resin, colour pigments and various additives are melt dispersed in a heating kneader. The preliminary dispersion composition of each original colour is mixed to obtain the desired colour, and the curing agent and optionally resins and additives are added to the mixture which is then melt mixed and pulverized.
However, these processes require the storage and production of appropriate finished coating compositions, e.g. pigmented coating powders or pigment-coated resins in a large number of different primary colours, in order to be able to produce the broadest range of final product colours. In addition, a uniform colour often cannot be obtained if the particle sizes of the different powder compositions to be mixed have too high value such as higher than, for example, 20 micrometers. Additionally, the production of such finely divided coating powders is energy-intensive and may create both dust explosion hazards and can also be inhaled by operators if not handled properly. Also, the coating powders will tend to absorb water if not stored properly, and coating powders with variable water content cannot be accurately measured in order to achieve a desired product colour, and agglomerated coating powders cannot be mixed as efficiently in order to produce a high quality final product.
JP 2001-288414 refers to a method for producing a powder coating which is suitable for the production of small batches of various colours using a few types of primary colour pellets and dry-mixing the types of pellets in suitable amounts to create the pre-determined coating colour followed by co-milling thereof. The powder pellets are produced by mixing, extruding and grinding of powder coating components with pigments and additives to obtain pellets having a specific colour.
WO 2006/047238 discloses the use of liquid pigment dispersions in which pigment is dispersed in a liquid polyester resin or optionally in a dispersing resin and solvent. This approach could not produce a high quality consistent product without multiple intermediate colour checks and adjustments to the powder coating during processing. For example, blue formulations could show weaker colour strength in a polyurethane (PUR) chemistry, but stronger colour strength in hybrid chemistry, compared to a traditional formulation. The converse could be true for formulations using a red pigment. Thus it is not proven that the use of pigmented liquid dispersions described led to improved colour strength in the final powder coating. Additionally, the use of liquid dispersing resins having a low glass transition temperature necessitates the use of further additives in the powder coating formulation to increase the glass transition temperature of the final product to give the required storage stability for the coating powder.
WO 2007/087169 and WO 2007/140131 refer to dispersions of polymer-enclosed colour-imparting particles for incorporating into powder coating compositions. The particles need to be prepared by polymerisation enclosure.